Circuit substrate material, circuits comprising the same, and method of manufacture thereof

ABSTRACT

An electrical circuit material having a conductive layer disposed a substrate, wherein the substrate is formed from a thermosetting composition comprising a polybutadiene or polyisoprene resin; an optional, functionalized liquid polybutadiene or polyisoprene resin; an optional butadiene- or isoprene-containing copolymer; an optional low molecular weight polymer; an optional curing agent; a cross-linking agent; a particulate fluoropolymer; and about 20 to about 50 percent by weight, based on the total weight of the thermosetting composition, of a magnesium hydroxide having a low ionic content. Use of magnesium hydroxide allows the composition to attain a high level of flame retardancy without use of halogenated flame retardants, while maintaining good moisture absorption and other physical properties.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefits of U.S. Provisional PatentApplication Ser. No. 60/423,281 filed Nov. 1, 2002, which is fullyincorporated herein by reference.

BACKGROUND

This disclosure relates generally to a method of making thermosettingcompositions for use in electrical circuit materials and the resultingproducts, and in particular to thermosetting polybutadiene andpolyisoprene circuit substrate materials.

As used herein, a circuit material is an article used in the manufactureof circuits and multi-layer circuits, and includes circuit laminates,bond plies, resin coated conductive layers, and cover films. Circuitlaminates, bond plies, resin coated conductive layers, and cover filmsin turn are formed from dielectric materials that can comprise athermosetting or thermoplastic polymer. The dielectric material in abond ply, resin covered conductive layer, or cover film may comprise asubstantially non-flowable dielectric material, i.e., one that softensor flows during manufacture but not use of the circuit, whereas thedielectric material in a circuit laminate (e.g., a dielectric substrate)is designed to not soften or flow during manufacture or use of thecircuit or multi-layer circuit. Dielectric substrate materials arefurther typically divided into two classes, flexible and rigid. Flexibledielectric substrate materials generally tend to be thinner and morebendable than the so-called rigid dielectric materials, which typicallycomprise a fibrous web or other forms of reinforcement, such as short orlong fibers or fillers.

A circuit laminate as used herein refers to one or two conductive layersfixedly attached to a dielectric substrate, which is formed from adielectric material. Patterning a conductive layer of a laminate, e.g.,by etching, provides a circuit. Multi-layer circuits comprise aplurality of conductive layers, at least one of which contains aconductive wiring pattern. Typically, multi-layer circuits are formed bylaminating one or more circuits together using bond plies, and, in somecases, resin coated conductive layers, in proper alignment using heatand/or pressure. The bond plies are used to provide adhesion betweencircuits and/or between a circuit and a conductive layer, or between twoconductive layers. In place of a conductive layer bonded to a circuitwith a bond ply, the multi-layer circuit may include a resin coatedconductive layer bonded directly to the outer layer of a circuit. Insuch multi-layer structures, after lamination, known hole forming andplating technologies may be used to produce useful electrical pathwaysbetween conductive layers.

Polybutadiene and polyisoprene thermosetting materials have beensuccessfully employed as rigid electrical circuit substrates. Thesematerials have typically used halogenated, particularly brominated,flame retardant additives to achieve the necessary levels of flameretardancy. In recent years, brominated flame retardants have come underscrutiny, such that certain of them will be banned by January 2008. Theremaining brominated flame retardants will require specialincineration/disposal procedures. In light of the impending ban,manufacturers are placing additional pressures upon suppliers to produceflame retardant additives that are effective, yet that do not containhalogens.

The most commonly used alternative flame retardant additives arephosphorous/nitrogen compounds. However, phosphorous/nitrogen compoundspossess high dielectric constants, loss factors, and moisture absorptionproperties. These properties are adverse to intended uses inapplications such as the electronic industries, automobile industries,and particularly in circuit boards and related applications.Accordingly, there remains a need for non-halogen containing flameretardant thermosetting compositions that provide the desired flameretardant properties without impairing physical properties such aselectrical and moisture absorption properties.

SUMMARY

The above discussed and other problems and deficiencies of the prior artare overcome or alleviated by a substrate for an electrical circuitmaterial, wherein the substrate is formed from a thermosettingcomposition comprising a polybutadiene or polyisoprene resin; anoptional, functionalized liquid polybutadiene or polyisoprene resin; anoptional butadiene- or isoprene-containing copolymer; an optional lowmolecular weight polymer; a cross-linking agent; a particulatefluoropolymer; and about 20 to about 50 percent by weight, based on thetotal weight of the thermosetting composition, of a magnesium hydroxidehaving less than about 1000 ppm of ionic contaminants, and furtherwherein the substrate has a UL-94 rating of at least V-1.

In another embodiment, an electrical circuit material comprises asubstrate and a layer of conductive metal disposed on the substrate,wherein the substrate is formed from a thermosetting compositioncomprising a polybutadiene or polyisoprene resin; an optional,functionalized liquid polybutadiene or polyisoprene resin; an optionalbutadiene- or isoprene-containing copolymer; an optional low molecularweight polymer; a cross-linking agent; a particulate fluoropolymer; andabout 20 to about 50 percent by weight, based on the total weight of thethermosetting composition, of a magnesium hydroxide having a low ioniccontent, and further wherein the substrate has a UL-94 rating of atleast V-1.

Use of magnesium hydroxide in the thermosetting compositions allows thesubstrate materials to achieve a desired flame retardancy without use ofbrominated flame retardants, but does not adversely affect propertiessuch as dielectric constant or moisture absorbance. The above-discussedand other features and advantages will be appreciated and understood bythose of ordinary skill in the art from the following detaileddescription and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings, wherein like elements are numbered alikein the several FIGURES:

FIG. 1 is a schematic representation of a diclad circuit materialcontaining a woven web; and

FIG. 2 is a schematic representation of a circuit material.

DETAILED DESCRIPTION

A thermosetting composition having particular utility as a substrate foran electrical circuit material comprises a thermosetting polybutadieneor polyisoprene resin system; about 20 to about 50 percent by weight,based on the total weight of the composition, of magnesium hydroxidehaving a low ionic content; a crosslinking agent; and a particulatefluoropolymer.

Use of magnesium hydroxide as a flame retardant agent allows the circuitmaterial to achieve the desired flame retardancy in the absence ofhalogenated flame retardants, particularly brominated flame retardants.Dielectric properties are also acceptable with use of magnesiumhydroxide. This result is particularly surprising in view of the factthat magnesium hydroxide has a high dielectric constant and the presenceof such large amounts of magnesium hydroxide would have been expected togreatly increase the dielectric constant of the composition. However,the inventors hereof have discovered that use of a particularpolybutadiene or polyisoprene thermosetting composition, together withabout 20 to about 50 percent by weight of a magnesium hydroxide having alow ionic content, produces a thermosetting composition exhibitingexcellent flame retardancy, electrical and moisture resistanceproperties.

A number of commercially available magnesium hydroxides are suitable foruse in the present thermosetting compositions, for example thoseavailable under the trade name MAGNIFIN® from Albemarle Corp. Accordingto the product literature, MAGNIFIN® H51V and MAGNIFIN® H10IV aremagnesium hydroxides having a low ionic content, and are treated(coated) with an aminosilane. Low ionic content is herein defined ascontaining less than about 1,000 ppm, preferably less than about 500 ppmby weight of ionic contaminants such as chloride ion. In addition, it ispreferred that the magnesium hydroxide has a low total metal content,herein defined as less than about 500 ppm, preferably less than about400 ppm, more preferably less than about 300 ppm by weight of metalcontaminants such as iron, aluminum, chromium, manganese, copper, andthe like. It is especially preferred that the amount of iron oxide belimited to less than about 100 ppm, preferably less than about 50 ppm.Other suitable magnesium hydroxides are commercially available under thetrade name MAGSHIELD from Martin Marietta Corp., ZEROGEN from J. M.Huber Engineering materials, and FR20 from Dead Sea Bromine Group.

In addition, the particulate size of the magnesium hydroxide can impactthe electrical and flame retardant properties of the substrate material.Preferably the magnesium hydroxide has an average surface area (BET) ofabout 3 to about 12 square meters per gram, preferably about 5 to about10, and an average particle size of about 0.1 to about 2 micrometers.The magnesium hydroxide comprises about 20 to about 50 percent by weightof the total thermosetting composition (i.e., resin system as describedbelow, curing agent, crosslinking agent, particulate fluoropolymer, andmagnesium hydroxide, but exclusive of any reinforcing glass web orfiller).

The magnesium hydroxide is used in combination with a polybutadiene orpolyisoprene thermosetting resin system comprising (1) a polybutadieneor polyisoprene resin; (2) an optional, functionalized liquidpolybutadiene or polyisoprene resin; (3) an optional butadiene- orisoprene-containing copolymer; and (4) an optional low molecular weightpolymer.

As a first component, the resin system comprises a polybutadiene resin,a polyisoprene resin, or mixture thereof. The polybutadiene orpolyisoprene resins may be liquid or solid at room temperature. Liquidresins may have a molecular weight greater than about 5,000, butpreferably have a molecular weight of less than about 5,000 (mostpreferably between about 1,000 and about 3,000). The preferably liquid(at room temperature) resin portion maintains the viscosity of thecomposition at a manageable level during processing to facilitatehandling, and it also crosslinks during cure. Polybutadiene andpolyisoprene resins having at least 90% 1,2-addition by weight arepreferred because they exhibit the greatest crosslink density upon cureowing to the large number of pendant vinyl groups available forcrosslinking. High crosslink densities are desirable because theproducts exhibit superior performance in an electrochemical cellenvironment at elevated temperatures. A preferred resin is B3000 resin,a low molecular weight polybutadiene liquid resin having greater than 90weight percent (wt. %) 1,2-addition. B3000 resin is commerciallyavailable from Nippon Soda Co., Ltd.

The resin system further optionally comprises a functionalized liquidpolybutadiene or polyisoprene resin. Examples of appropriatefunctionalities for butadiene liquid resins include but are not limitedto epoxy, maleate, hydroxy, carboxyl and methacrylate. Examples ofuseful liquid butadiene copolymers are butadiene-co-styrene andbutadiene-co-acrylonitrile. Possible functionalized liquid polybutadieneresins include Nisso G-1000, G-2000, G-3000; Nisso C-1000; NissoBN-1010, BN-2010, BN-3010, CN-1010; Nisso TE-2000; and Nisso BF-1000commercially available from Nippon Soda Co., Ltd. and Ricon 131/MAcommercially available from Colorado Chemical Specialties, Inc.

The optional, butadiene- or isoprene-containing copolymer is preferablyunsaturated and can be liquid or solid. It is preferably a solid,thermoplastic elastomer comprising a linear or graft-type blockcopolymer having a polybutadiene or polyisoprene block, and athermoplastic block that preferably is styrene or 1-methyl styrene.Possible block copolymers, e.g., styrene-butadiene-styrene tri-blockcopolymers, include Vector 8508M (commercially available from DexcoPolymers, Houston, Tex.), Sol-T-6302 (commercially available fromEnichem Elastomers American, Houston, Tex.), and Finaprene 401(commercially available from Fina Oil and Chemical Company, Dallas,Tex.). Preferably, the copolymer is a styrene-butadiene di-blockcopolymer, such as Kraton D1118X (commercially available from ShellChemical Corporation). Kraton DI 118X is a di-block styrene-butadienecopolymer containing 30 vol % styrene.

The butadiene- or isoprene-containing polymer may also contain a secondblock copolymer similar to the first except that the polybutadiene orpolyisoprene block is hydrogenated, thereby forming a polyethylene block(in the case of polybutadiene) or an ethylene-propylene copolymer (inthe case of polyisoprene). When used in conjunction with the firstcopolymer, materials with enhanced toughness can be produced. Where itis desired to use this second block copolymer, a preferred material isKraton GX1855 (commercially available from Shell Chemical Corp.), whichis believed to be a mixture of styrene-high 1,2-butadiene-styrene blockcopolymer and styrene-(ethylene-propylene)-styrene block copolymer.

Thus, in a preferred embodiment, the butadiene- or isoprene-containingpolymer comprises a solid thermoplastic elastomer block copolymer havingthe formula X_(m)(Y—X)_(n) (linear copolymer) or

(graft copolymer), where Y is a polybutadiene or polyisoprene block, Xis a thermoplastic block, and m and n represent the average blocknumbers in the copolymer, m is 0 or 1 and n is at least 1. Thecomposition may further include a second thermoplastic elastomer blockcopolymer having the formula W_(p)(Z-W)_(q) (linear copolymer) or

(graft copolymer) where Z is a polyethylene or ethylene-propylenecopolymer block, W is a thermoplastic block, and p and q represent theaverage block numbers in the copolymer, p being 0 and 1 and q being atleast 1.

The volume to volume ratio of the polybutadiene or polyisoprene resin tobutadiene- or isoprene-containing polymer preferably is between 1:9 and9:1, inclusive. The selection of the butadiene- or isoprene-containingpolymer depends on chemical and hydrolysis resistance as well as thetoughness conferred upon the molded material.

The optional low molecular weight polymer resin is generally employed toenhance toughness and other desired characteristics of composition. Bylow molecular weight polymer, it is meant a polymer having a molecularweight of less than about 50,000, preferably less than about 5, 000.Examples of suitable low molecular weight polymer resins include, butare not limited to, telechelic polymers such as polystyrene,multifunctional acrylate monomers and ethylene propylene diene monomer(EPDM) containing varying amounts of pendant norbornene groups and/orunsaturated functional groups. The optional low molecular weight polymerresin can be present in amounts of zero to about 30 wt % of the resinsystem.

A curing agent may be used to accelerate the curing reaction. When thecomposition is heated, the curing agent decomposes to form freeradicals, which then initiate cross linking of the polymeric chains.Preferred curing agents are organic peroxides such as Luperox, dicumylperoxide, t-butyl perbenzoate, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, α,α-bis(t-butyl peroxy)diisopropylbenzene, and2,5-dimethyl-2,5-di(t-butyl peroxy) hexyne-3, all of which arecommercially available. They may be used alone or in combination.Typical amounts of curing agent are about 1.5 wt % to about 6 wt % ofthe resin system.

Crosslinking agents may also be added to increase the crosslink densityof the resin(s). Examples of preferred cross-linking agents includetriallylisocyanurate, triallylcyanurate, diallyl phthalate, divinylbenzene, and multifunctional acrylate monomers (e.g., the Sartomerresins available from Arco Specialty Chemicals Co.), and combinationsthereof, all of which are commercially available, withtriallylisocyanurate being generally preferred. The cross-linking agentcontent of the thermosetting composition can be readily determined byone of ordinary skill in the art, depending upon the desired flameretardancy of the composition, the amount of the other constituentcomponents, and the other properties desired in the final product.UL-94, an Underwriters Laboratories flammability test, provides fourpossible ratings, HB, V-2, V-1, and V-0. V-0 is the most difficultrating to obtain, requiring that five bars of material self extinguishwith an average flame out time of five seconds or less without dripping.More particularly, the amount of cross-linking agent depends upon theloading of magnesium hydroxide and amount(s) of the other components inthe thermosetting composition, and attaining excellent flame retardancy,electrical and moisture properties. In general, effective quantities aregreater than or equal to about 0.5 wt %, preferably greater than orequal to about 1 wt %, and most preferably greater than or equal toabout 5 wt % based on the total weight of the thermosetting composition.Effective quantities are typically less than about 15 wt %, preferablyabout 10 wt %, and most preferably about 8 wt % based on the totalweight of the resin system.

Suitable particulate fluoropolymers for inclusion in the thermosettingcomposition include those known in the art for circuit substrates, andinclude but are not limited to fluorinated homopolymers, for examplepolytetrafluoroethylene (PTFE), and fluorinated copolymers, e.g.copolymers of tetrafluoroethylene with hexafluoropropylene orperfluoroalkylvinylethers such as perfluorooctylvinyl ether, orcopolymers of tetrafluoroethylene with ethylene. Blends of fluorinatedpolymers, copolymers, and terpolymers formed from the above listedmonomers are also suitable for use with the present invention. Aparticularly preferred fluoropolymer is PTFE.

Useful forms of particulate fluoropolymer resin include fine powder andgranular fluoropolymer, both of which are widely commercially available.As used herein, granular and dispersion are terms of art commonly usedin connection with the forms of fluoropolymers, and refers to thephysical characteristics of the fluoropolymer, particularly particlesize. Particle size in turn is determined by the method of fluoropolymermanufacture.

Fine powder PTFE (or coagulated dispersion) is made by coagulation anddrying of dispersion-manufactured PTFE. Fine powder PTFE is generallymanufactured to exhibit a particle size of approximately 400 microns to500 microns. It is used in the manufacture of paste extruded articles,such as wire insulation and in paste extrusion and calendaring.

Granular PTFE is made by a suspension polymerization method. GranularPTFE is generally used for compression molding of PTFE articles. It isalso widely used for the molding of billets that are then skived on alathe to produce PTFE sheet. Granular PTFE is generally manufactured intwo different particle size ranges. The standard product is made with amedian particle size of approximately 30 microns to 40 microns. The highbulk density product exhibits a median particle size of about 400microns to 500 microns. In addition to these forms of PTFE, otherfluoropolymer compositions such as DuPont Teflon FEP and PFA are alsoavailable in pellet form. The pellets can be cryogenically ground toexhibit a median particle size of less than 100 microns (μm). It isexpected that such materials, with the appropriate particle sizedistribution would act to achieve the same end in the present inventionas granular PTFE. Accordingly, granular fluoropolymers as used hereinmay refer to fluoropolymers manufactured by either suspensionpolymerization or by cryogenic grinding of pellets to the granular form.Particularly preferred for use is a granular PTFE commercially availableunder the trade name Zonyl MP-1100, available from DuPont, Wilmington,Del. having a particle size on the order of 35 microns.

The optimal particulate fluoropolymer content of the thermosettingcomposition can be readily determined by one of ordinary skill in theart, depending upon the necessary flame retardancy of the composition,for example, based on V-0, V-1 or V-2 in UL-94, the amount of the otherconstituent components, and the other properties desired in the finalproduct. More particularly, the amount of the fluoropolymer compositiondepends upon the loading of magnesium hydroxide and amount(s) of othersynergists in the thermosetting composition, and attaining excellentflame retardancy, electrical and moisture properties. In general,effective quantities are greater than or equal to about 1 phr,preferably greater than or equal to about 5 phr, and most preferablygreater than or equal to about 10 phr (parts per hundred parts byweight) of the total thermosetting resin composition. Effectivequantities are typically less than or equal to about 90 phr, preferablyless than or equal to about 75 phr, and most preferably less than orequal to about 50 phr of the total weight of the thermosettingcomposition.

Use of magnesium hydroxide as disclosed herein can eliminate the needfor a halogenated flame retardant, and at the very least, allows use ofadvantageously low levels of such flame retardants. The thermosettingcomposition thus optionally comprises a halogenated flame retardant. Thehalogenated flame retardant can comprise less than or equal to about 900parts per million (ppm) of a chlorine-containing flame retardant andless than or equal to about 900 ppm of a bromine-containing flameretardant, for a total halogenated flame retardant concentration of 1800ppm, based on the resin system, preferably based on the totalthermosetting composition (resin system plus magnesium hydroxide), morepreferably based on the total dielectric material (resin system plusmagnesium hydroxide plus particulate inorganic filler plus woven ornon-woven web). A suitable bromine-containing flame retardant isethylenebistetrabromopthalimide available as Saytex BT-93 fromAlbermarle Corp. A suitable chlorine-containing flame retardant is1,4:7,10-dimethanodibenzo (a,e) cyclooctane1,2,3,4,7,8,9,10,13,13,14,14-dodecachloro-1,4,4a,5,6,6a,7,10,10a,11,12,12a-dodecahydro,which is available as Dechlorane Plus from OxyChem.

The thermosetting composition optionally comprises a filler. Preferably,the filler material and quantity thereof is selected so as to providethe substrate with a coefficient of thermal expansion that is equal orsubstantially equal to the coefficient of thermal expansion of the metallayer. Suitable fillers include, for example, rutile titanium dioxideand amorphous silica because these fillers have a high and lowdielectric constant, respectively, thereby permitting a broad range ofdielectric constants combined with a low dissipation factor to beachieved in the final cured product by adjusting the respective amountsof the two fillers in the composition. To improve adhesion between thefillers and resin, coupling agents, e.g., silanes, can be used.

The volume percent (vol %) of the filler (based upon the combined volumeof the resin system, and particulate filler) is about 5% to about 60%,preferably about 30% to about 50%. Examples of preferred fillers includetitanium dioxide (rutile and anatase), barium titanate, strontiumtitanate, silica (particles and hollow spheres) including fusedamorphous silica and fumed silica; corundum, wollastonite, aramidefibers (e.g., Kevlar), fiberglass, Ba₂Ti₉O₂₀, glass spheres, quartz,boron nitride, aluminum nitride, silicon carbide, beryllia, alumina ormagnesia. They may be used alone or in combination.

A very high surface area particulate filler such as fumed silica may beadditionally used to prevent tackiness and stickiness in the prepreg.The preferred fumed silica is available from Degussa under the tradename AEROSIL 200, and has a surface area of about 200 m²/g, with atypical primary particle size of about 12 nanometers. The amount offumed silica used may be about 0.2 to about 5 vol %, and preferablyabout 0.5 to about 1.5 vol %.

The compositions optionally comprise woven, thermally stable webs of asuitable fiber, preferably glass (E, S, and D glass) or high temperaturepolyester fibers (e.g., KODEL from Eastman Kodak). The web is present inan amount of about 10 to about 40 vol %, and preferably about 15 toabout 25 vol % with respect to the thermosetting resin composition. Suchthermally stable fiber reinforcement provides the laminate with a meansof controlling shrinkage upon cure within the plane of the laminate. Inaddition, the use of the woven web reinforcement renders a dielectricsubstrate with a relatively high mechanical strength. In general, thethermosetting composition is processed as follows. First, all thecomponents (resin, magnesium hydroxide, cross-linking agent, particulatefluoropolymer and other desired additives) are thoroughly mixed inconventional mixing equipment along, preferably with a peroxide curingagent. The mixing temperature is regulated to avoid substantialdecomposition of the curing agent (and thus premature cure). Mixingcontinues until the ingredients are uniformly dispersed throughout theresin. For those applications where the thermosetting composition is toimpregnate a woven web forming a prepreg, conventional prepregmanufacturing methods can be employed. Typically the web is impregnatedwith the slurry, metered to the correct thickness, and then the solventremoved to form a prepreg.

The lamination process entails a stack-up of one or more saturated wovenwebs (or a non-saturated woven web sandwiched between two bond plies)between one or two sheets of conductive foil (copper). The stack-up isthen cured (via lamination) in a one step or two step curing cycle.

The stack-up can be cured using a conventional peroxide cure step attemperatures between about 150° C. and about 200° C. The peroxide-curedstack-up may then be subjected to a high-energy electron beamirradiation cure (E-beam cure) or a high temperature cure step under aninert atmosphere to impart an unusually high degree of cross-linking tothe resulting laminate. The temperature is greater than about 250° C.but less than about 400° C., or the decomposition temperature of theresin. This high temperature cure is preferably carried out in an ovenbut can also be performed in a press, namely as a continuation of theinitial lamination step.

In the alternative, the thermosetting composition can be mixed with asolvent to form a casting composition. The casting composition isapplied to a substrate. Thereafter, the solvent is removed and the castresin system is subjected to the aforementioned cure cycle.

In one preferred embodiment, the thermosetting composition includes aplurality of woven webs (such as E-glass webs) embedded in a mixture ofthe polybutadiene or polyisoprene based resin system and inorganicfiller (e.g., silica) laminated between one or two sheets of conductivefoils (e.g., copper) to produce a circuit board material that isespecially well suited for microwave applications. Of course, if verythin (e.g., less than 5 mil thickness) cross-sections are desired, thenonly a single saturated web may be used for the dielectric layer.Referring now to FIG. 1, a cross-sectional view of a circuit materialcomprising the thermosetting composition is shown generally at 10.Circuit substrate 10 has been laminated in accordance with one of theprocesses described above wherein a woven web 12 is embedded in athermosetting composition as described herein 14 and laminated betweentwo conductive layers 16, 18, for example copper foils, to produce acircuit material. As discussed above with reference to the processingconditions, the thermosetting composition 14 may either be cast ontowoven web 12 using known casting equipment, or woven web 12 may besaturated by thermosetting composition 14 by sandwiching woven web 12between a pair of bond plies formed from thermosetting composition 14and laminating the stack up together with the conductive layers 16, 18.While FIG. 1 depicts a single layer of woven web 12, it will beappreciated that typically a plurality of layers of saturated woven web12 will be used in forming circuit laminates. However, a single layer asshown in FIG. 1 is desirable where very thin cross-sections, e.g., lessthan about 5 mils, are required.

In FIG. 2 is shown a circuit material 20 formed from a dielectricmaterial 22 comprising a flame retardant thermosetting polybutadiene orpolyisoprene resin composition as described herein disposed on aconductive layer 24, such as copper. Any one or more of conductivelayers 16, 18, or 24 may be etched by known methods to provide a circuitlayer.

The thermosetting composition described above has numerous advantages. AUL-94 rating of V-1 or better may be obtained without the use ofhalogenated flame retardants. In addition a dielectric constant (Dk)less than about 4.5 and preferably less than about 4.0 may be obtained.The dissipation or dielectric loss factor (Df) is less than about 0.01and preferably less than about 0.006. Finally the moisture absorption ofthe thermosetting composition is less than about 0.2 and preferably lessthan about 0.15%.

The following non-limiting examples further describe the thermosettingcomposition.

EXAMPLES

Compositions as described in U.S. Pat. No. 6,048,807, comprising apolybutadiene resin (B3000 from Nisso) and a cure agent, together withthe additives as shown with Table 1, were formulated and tested. Ingeneral, the compositions were processed as follows. First, thepolybutadiene resin, magnesium hydroxide, and all other components werethoroughly mixed to form a slurry in conventional mixing equipment. Themixing temperature was regulated to avoid substantial decomposition ofthe curing agent (and thus premature cure). Next, conventional prepregmanufacturing methods were employed. Typically, if used, the web wasimpregnated with the slurry, metered to the correct thickness, and thenthe solvent was removed (evaporated) to form a prepreg. The laminationprocess entailed a stack-up of one or more prepreg layers between one ortwo sheets of conductive foil (copper). This stack-up was then densifiedand cured via lamination or a combination of lamination and oven baking.The stack-up was cured in a conventional peroxide cure step; typicalcure temperatures were between about 330 and about 425° F. (about 165 toabout 218° C.).

In the following examples, flame retardance was measured in accordancewith UL-94. The designation “fail” indicates that the sample did notattain V-1.

Dielectric constant (Dk) values are the averages of the measureddielectric constants from a 1-10 Ghz frequency sweep.

Dissipation Factor (Df) values are the lowest recorded value of a given1-10 Ghz frequency sweep.

Specific gravity (“Sp.g.”) was determined in accordance with ASTMD79291.

Water absorption was measured by IPCTM-650 2.6.2.1 (with 48 hrexposure). TABLE 1 Example No. 1* 2* 3* 4 5 6 7** Component (wt %)Magnesium hydroxide 50 20 25 20 (Magnifin H10) Magnesium hydroxide 38 3550 (Zerogen) Magnesium hydroxide 30 (Magnifin H5A) Magnesium hydroxide15 20 (Magnifin H5) Magnesium hydroxide 10 10 (Magnifin H3) Silica 18 2528 8 15 15 15 Triallylisocyanurate 5 5 5 5 5 Particulate PTFE 5 5 5 5 5Flame retardant (Saytex BT-93) 0.2 0.2 0.2 Flame retardant (Dechlorane0.2 0.2 0.2 plus) Results UL-94 Fail Fail Fail V-1 V-0 V-0 V-0 Df at 3GHz 0.0047 0.0053 0.0054 0.0061 0.0047 0.0047 0.0047 Dielectric constant3.93 3.93 4.06 4.01 3.83 3.94 Water absorbance 0.2 0.25 0.11 0.17 0.080.12 Specific gravity 1.88 1.88 1.88*Denotes comparative examples**Post cured with Perkadox

Comparative Example 1 has magnesium hydroxide only, Comparative Example2 has a combination of magnesium hydroxide and triallylisocyanuratecross-linker, and Comparative Example 3 has a combination of magnesiumhydroxide and polytetrafluoroethylene. All of the comparative examplesfailed the UL-94 test. In addition, the samples had dissipation factorsless than about 0.006, dielectric constants less than about 4.5, waterabsorption of less than about 0.2, and xylene absorption (data notshown) of less than about 1.1.

Example 4 with magnesium hydroxide, triallylisocyanurate cross-linker,and polytetrafluoroethylene had a UL 94 rating of V-1. Thus, thecombination of three additives provides a V-1 rating in the absence ofadditional flame retardants.

Examples 5-7 contain fire retardants in addition to the magnesiumhydroxide, triallylisocyanurate cross-linker, andpolytetrafluoroethylene. All three examples achieve a V-0 rating. Inaddition, the samples had dissipation factors less than about 0.005,dielectric constants less than about 4.1, and water absorption of lessthan about 0.2. Example 5 contains a combination of uncoated (MagnifinH10) and coated (Magnifin H5A) magnesium hydroxides. The coatedmagnesium hydroxide may be optionally used to prevent loss of magnesiumhydroxide during acidic processing conditions. The different examplesalso contain magnesium hydroxide particles having different surfaceareas. Comparing Examples 5, 6 and 7, Example 5 has a combination of 10square meters per gram surface area and 5 square meters per gram surfacearea sized magnesium hydroxide, while Examples 6 and 7 have acombination of 10, 5 and 3 square meters per gram surface area (MagnifinH10, H5 and H, respectively). The particle surface area may be variedwhen it is desirable to modify the particle packing fraction.

In summary, the data show thermosetting compositions with UL-94 ratingsof V-0 and V-1, with Example 4 achieving a V-1 rating without use ofadditional flame retardant. In addition, dielectric constants of lessthan about 4.5 and even less than about 4.0 can be achieved. Thedissipation factors of the compositions are less than about 0.0065.Also, the water absorption is less than about 2.5 with many samples lessthan 0.2. The data thus clearly show that the thermosetting compositionscomprising magnesium hydroxide, PTFE, and crosslinker possess acceptableUL-94 ratings, dielectric constants, dissipation factors, and moistureabsorption properties with and without the addition of halogenated flameretardants.

While preferred embodiments have been shown and described, variousmodifications and substitutions may be made thereto without departingfrom the spirit and scope of the invention. Accordingly, it is to beunderstood that the present invention has been described by way ofillustration and not limitation.

1. A method of making an electrical circuit substrate material, themethod comprising disposing onto a conductive layer a thermosettingcomposition comprising a polybutadiene or polyisoprene resin; across-linking agent; a particulate fluoropolymer; and about 20 to about50 percent by weight, based on the total weight of the thermosettingcomposition, of a magnesium hydroxide having less than about 1000 ppm ofionic contaminants; and curing the thermosetting composition wherein thesubstrate has a UL-94 rating of at least V-1.
 2. The method of claim 1,wherein the thermosetting composition further comprises a butadiene- orisoprene-containing copolymer.
 3. The method of claim 2, wherein thebutadiene- or isoprene-containing copolymer is an unsaturated butadiene-or isoprene-containing copolymer.
 4. The method of claim 3, wherein thevolume to volume ratio of the polybutadiene or polyisoprene resin to theunsaturated butadiene- or isoprene-containing copolymer is between 1:9and 9:1, inclusive.
 5. The method of claim 1, wherein the thermosettingcomposition further comprises a curing agent.
 6. The method of claim 5,wherein the curing agent is an organic peroxide, a dicumyl peroxide, adi(2-tert-butylperoxyisopropyl) benzene, a t-butylperbenzoate, at-butylperoxy hexyne-3, or a combination comprising one or more of theforegoing curing agents.
 7. The method of claim 1, wherein thethermosetting composition further comprises a low molecular weightpolymer.
 8. The method of claim 1, wherein the thermosetting compositionfurther comprises a functionalized liquid polybutadiene or polyisopreneresin.
 9. The method of claim 1, wherein the cross-linking agent istriallylisocyanurate, triallylcyanurate, diallyl phthalate, divinylbenzene, a multifunctional acrylate monomer, or a combination comprisingone or more of the foregoing cross-linking agents.
 10. The method ofclaim 1, wherein the particulate fluoropolymer is a difluoroethylenepolymer, a tetrafluoroethylene polymer, atetrafluoroethylene-hexafluoropropylene copolymer, a copolymer oftetrafluoroethylene with fluorine-free ethylenic monomers, or acombination comprising one or more of the foregoing particulatefluoropolymers.
 11. The method of claim 1, wherein the substrate has amoisture absorption value less than about 0.2% and a UL-94 flammabilityrating of V-0.
 12. The method of claim 1, wherein the substrate has adielectric constant less than about 4.5 and a dielectric loss factorless than about 0.01.
 13. The method of claim 1, wherein the conductivelayer is copper.
 14. The method of claim 1, wherein the thermosettingcomposition further comprises a woven or non-woven glass web.
 15. Theelectrical circuit material of claim 1, wherein the magnesium hydroxidecomprises less than about 500 ppm of metal.
 16. The method of claim 1,wherein the thermosetting composition further comprises achlorine-containing flame retardant, a bromine-containing flameretardant, or a combination comprising one or more of the foregoingflame retardants.
 17. The method of claim 1, wherein the magnesiumhydroxide has an average surface area of about 3 to about 12 meterssquared per gram.
 18. The method of claim 1, wherein the magnesiumhydroxide is coated with an aminosilane.
 19. The method of claim 1,further comprising a filler.
 20. The method of claim 19, where thefiller further comprises a coupling agent.
 21. The method of claim 19,where the coupling agent is a silane.